The UK Foreign Office’s own risk assessment warned that the Taliban could return rapidly to power, causing cities to collapse and triggering a humanitarian crisis, less than four weeks before the fall of Kabul.

Dominic Raab, the foreign secretary, told MPs on Wednesday that he believed the Afghan capital would remain safe until next year based on an assessment by the joint intelligence committee, a view he said was widely shared by Nato allies.

In a two-hour grilling on the handling of the Afghanistan crisis following a fortnight of criticism, he admitted the UK had been “caught out and surprised by the scale and speed of the fall of Kabul”, accepting lessons would have to be learned from how the intelligence assessment was made.

A day after the US president, Joe Biden, said the era of US nation building was over, Raab also appeared to question how much support the British public had for overseas interventions.

During the hearing, Raab appeared to be taken by surprise when he was asked why he had not acted on the Foreign Office “principal risk report assessment” from 22 July, which warned: “Peace talks are stalled and US Nato withdrawal is resulting in rapid Taliban advances. This could lead to: fall of cities, collapse of security forces, Taliban return to power, mass displacement and significant humanitarian need. The embassy may need to close if security deteriorates.”

When Raab asked for the source, Tom Tugendhat, the chair of the foreign affairs select committee that questioned him, said: “It’s your principal risk report.”

A copy of the report seen by the Guardian reveals it went on to highlight potential consequences for the UK, including “reducing UK counter-terrorism capability and increasing compliance risks; enabling the flow of narcotics and illegal migration; increasing global humanitarian pressures; reducing oversight of government funding; destabilising the wider region, especially Pakistan; damaging the reputation of the UK and Nato and increasing threats to staff to whom [the Foreign Office] has a duty of care”. No timeline was set out, however.

MPs pressed Raab on whether the Foreign Office had failed to draw up more robust contingency plans before Kabul fell to the Taliban on 15 August, necessitating the mass evacuation of thousands of British nationals and officials.

In response to claims that he had been slow to grasp the scale of the crisis, including by leaving on holiday at a critical time, Raab revealed he was travelling to the region including Pakistan on Wednesday evening. He will first go to Doha in Qatar, where the Taliban have an office.

The Foreign Office said the unpublished document cited by Tugendhat was a corporate risk report to the department’s monthly board meeting based on open sources, and did not contradict the intelligence risk assessment.

The document informed the department’s board that “our risk profile remains largely stable but elevated; no principal risk is rising” before going on to single out Afghanistan as one area where there is “a major risk that existing violent conflict worsens”. It said the Foreign Office had already taken some actions, including reducing staffing at the embassy and bolstering diplomatic support for peace talks.

Asked by MPs why the central British intelligence assessment was so badly wrong, Raab first blamed “optimism bias” about how long the US might retain troops in the country, adding he personally cautioned about the Taliban’s intent.

But he continued: “In fairness, collectively across allies, the assessment that they would not be able to advance at that speed was not correct.” Asked for the source of this misjudgment, he said: “There is a desire and a determination to make it better, make it work and complete the task.”

The Labour MP Chris Bryant asked whether he was already on holiday on the Greek island of Crete on 11 August when the US was saying the Taliban was likely to take power. Raab repeated he “would not have gone away, with the benefit of hindsight” before saying: “I am not going to start adding to, frankly, the fishing expedition beyond the facts that I have articulated and the fulsome statement and having answered questions on this continuously.”

He said he had been involved in 40 calls that included Afghanistan between March and mid-August this year. He also revealed that since 2019 he had overseen direct back-channel talks between Afghanistan and Pakistan, chaired by the UK chief of the defence staff. But he admitted he had not travelled to Pakistan, Afghanistan or other neighbouring countries during his time as foreign secretary.

Asked about efforts to get thousands of people with British links out of Afghanistan, Raab said a “peak” of 20 Foreign Office staff were in the country during the evacuation effort, and they were joined by 13 Border Force officials.

Raab was criticised for the government setting up three different categories of people in Afghanistan eligible to settle in Britain, each run by a different government department: British nationals, those who have shown loyalty to the UK and an “asylum-related” group “based on international law”.

Asked why there was no triage system and why the categories were dealt with by different government departments, he said: “I think we’re doing everything we can, and the proof is in the 17,000 [people] that since April we have secured safe passage for back to the UK.”

Graham Stringer, a Labour MP, accused Raab of presiding over “a failure of planning on a grand scale, some are at risk, some have died, and all I have heard is that you wish you had not gone on holiday”. His colleague Neil Coyle asked: “Why is global Britain so isolated in comparison with Great Britain under Margaret Thatcher and Tony Blair?”

Raab repeatedly suggested “in the US domestic support for those kind of interventions has fallen away”, adding: “There is a question of what our polity, our public will be willing to support.” He said in the UK there had been “too much wishful thinking about the direction of the US debate about forever wars”.

Pressed again on why Nato concluded it did not have the capacity to continue the mission without the US, he said “there was a question of psychological confidence of operating without the US” but made no mention of coordination with European powers. “For all the criticism of the US over this, I am in no doubt that the US will bounce back. It is indispensable and we will learn the lessons together,” he said.